Pressure sensors are widely applied in IoT applications such as drones and industrial automation. However, designers are constantly challenged to improve the accuracy, precision, resolution, noise immunity, and temperature stability of their devices, at lower cost and with shorter time to market.
To address these challenges, sensor providers have introduced innovative new capabilities, form factors, integrated electronics, and flexible interface options that contribute significantly to design and integration simplicity.
This article will discuss the anatomy of modern integrated pressure sensors and how they deal with issues such as temperature compensation and output accuracy. It will discuss key design considerations while introducing suitable solutions, and how developers can quickly get up and running with them.
The evolution of pressure sensors
Pressure sensors started out as electromechanical devices, but those have given way to lower cost, semiconductor-based devices using MEMS that can measure extremely small pressure differences of ±1 pascal (Pa). With on-board interfaces, they can send data to a microcontroller via an I2C or SPI link while consuming much less power.
In a MEMS pressure sensor, force is applied to a flexible membrane that deflects on a sensing element to induce an imbalance that is transformed into an output. MEMS-based sensors measure both absolute and differential pressure ranges, and they come in both compensated and uncompensated versions.
Pressure sensors for IoT designs
Recent changes have made pressure sensors far more accurate, lighter, and lower cost, as well as enabling greater measurement ranges. These innovations are required by new applications in the IoT and wearable design realms.
For example, applications for MEMS-based pressure sensors include next generation sports bands seeking greater accuracy in calorie expenditure measurements. Runners and cyclists are very particular about improving the monitoring accuracy of their performance. The fact that pressure sensing is becoming a staple in wearable and IoT designs inevitably means it has to have a smaller footprint.
The new smaller and lower power MEMS sensors can significantly save board space while enhancing the performance and reliability of IoT designs. These pressure sensors in ultra-compact and thin packages also suit battery-driven portable designs such as smartphones and tablets, as well as sports wearables.
In some of these battery-powered mobile devices, pressure sensors are augmenting or replacing GPS in applications such as activity identification, accurate floor level detection, and outdoor localization. These MEMS-based pressure sensors also allow more accurate dead reckoning calculations, creating new applications in the areas of healthcare and weather monitoring.
A good example of these newer MEMS sensors include the LPS22HB from STMicroelectronics (Figure 1). It is a MEMS nano pressure sensor with an absolute pressure range of 260 to 1260 hectopascals (hPa) and a digital output. Key features include ultra-small dimensions of 2.0 x 2.0 x 0.76 millimeters (mm) in an LGA package and low power consumption, drawing only 3 microamps (µA) off a 1.7 volt to 3.6 volt supply.
Figure 1: STMicroelectronics’ LPS22HB MEMS barometer measures 2 x 2 x 0.76 mm and draws 3 µA off a 1.7 volt to 3.6 volt supply. (Image source: STMicroelectronics)
The LGA package is holed to allow external pressure to reach the sensing element. The sensor is piezoresistive and comprises the sensing element and an IC interface that links the sensing element to the application via I2C or SPI.
The LPS22HB features temperature and pressure compensation, and it contains a built-in FIFO to efficiently handle the pressure and temperature data in the digital logic (Figure 2).
Figure 2: The LPS22HB’s FIFO feature is contained within the digital logic portion, along with temperature and pressure compensation. (Image source: STMicroelectronics)
The FIFO buffer comprises 32 slots of 40-bit data to store the pressure and temperature output values. This enables consistent power savings as the host doesn’t have to keep polling the sensor. Instead, the host only has to wake up from an interrupt and burst the required data from the FIFO.
The FIFO has seven different operation modes; Bypass mode, FIFO mode, Stream mode, Dynamic-Stream mode, Stream-to-FIFO mode, Bypass-to-Stream mode, and Bypass-to-FIFO mode. These provide various levels of operability. For example, in Bypass mode it remains non-operational and empty, whereas Dynamic-Stream mode ensures the number of new data available in the FIFO does not depend on the previous reading.
When using the LPS22HB, power is supplied to pin 10 (VDD). It is recommended to put a 100 nanofarad (nF) decoupling capacitor as close as possible to the supply pads (Figure 3).
Figure 3: When laying the LPS22HB out on the pc board, connect pin 10 to VDD for the power sourcing, and place a 100 nF decoupling capacitor as close as possible to the supply pads. (Image source: STMicroelectronics)
Also, when using the I2C interface, CS (pin 6) must be tied to VDD_IO (pin 1).
Filtering out noise and sudden changes
To use pressure sensors in sophisticated designs such as smartwatches and sports bands, maintaining ultra-low noise is essential. This is especially so given the likelihood of abrupt events that cause a quick and sudden increase in barometric pressure.
To deal with noise, Bosch Sensortec’s BMP388 barometric pressure sensor includes an infinite impulse response (IIR) filter (Figure 4). This allows the pressure sensor to filter out sudden changes in pressure caused by environmental events.
Figure 4: The IIR filter in Bosch Sensortec’s BMP388 sensor facilitates low noise response to events like a door closing or a gunshot. (Image source: Bosch Sensortec)
The BMP388 is designed for altitude tracking in smartphones, smartwatches, and consumer drones. The low noise 24-bit absolute barometric pressure sensor offers a wide measurement range of 300 hPa to 1,250 hPa, and a relative accuracy of ±0.66 m (Figure 5).
Figure 5: Bosch-Sensortec's BMP388 digital MEMS barometric pressure sensor measuring 2 x 2 x 0.8 mm is designed to provide altitude information to navigation instruments with an accuracy of ±0.66 m.
If the barometric sensor cannot provide altitude stabilization in dynamic conditions such as sudden temperature fluctuations, barometric data can be combined with accelerometer data alongside a complementary filter. For such instances where sensor fusion techniques are required for optimum performance, Bosch Sensortec offers the BMI088 inertial measurement unit (IMU) for accurate steering, and the BMM150 geomagnetic sensor for the provision of heading data.
Measuring pressure under temperature extremes
Accuracy and resolution go hand-in-hand with pressure sensor design. However, pressure sensors have to be able to respond accurately in altitudes ranging from deep within mines to the tops of mountains, with the corresponding variations in temperature extremes. This, while also maintaining wet-media compatibility.
In applications such as drones, altitude information is crucial for stability and landing accuracy, but pressure sensors need to provide this altitude information with a high degree of accuracy and resolution, despite the changing environment. Temperature compensation using proprietary algorithms helps MEMS devices achieve an accuracy of ±1 Pa, which translates to altitude changes of less than 5 cm.
Temperature stability is especially crucial in always-on motion sensing applications like wearable devices where temperatures quickly shift as users move from one environment to another. NXP Semiconductors’ MPL3115A2 is a good example of how it’s accomplished (Figure 6).
Figure 6: A view of how pressure and temperature sensing operations complement each other in NXP's MPL3115A2 compact piezoresistive absolute pressure sensor. (Image source: NXP Semiconductors)
The MPL3115A2 has a wide operating range of 20 kPa to 110 kPa, a range that NXP has designed to cover all surface elevations on earth. It is temperature compensated using an on-chip temperature sensor, with both the pressure and temperature then being multiplexed, amplified, filtered and fed to an analog-to-digital converter (ADC). The altitude is then calculated using the formula in Equation 1:
h = altitude, given in meters and fractions of a meter
p0 = sea level pressure (101,326 Pa)
OFF_H = user input of the equivalent sea level pressure to compensate for local weather
conditions and the US Standard Atmosphere 1976 (NASA)
p = pressure, in Pa and fractions of a Pa.
Core specifications for the MPL3115A2 include on-chip processing to avoid taxing the host processor, and a typical active supply current of 40 µA per measurement-second for a stable output resolution. It operates off a 1.95 volt to 3.6 volt supply (internally regulated), and has an operating temperature range of -40˚C to +85˚C.
The diversity of application scenarios and conditions are well covered by sensor providers. Honeywell's TBF Series is a case in point. They’re a basic force sensor comprising a flush diaphragm pressure sensor that is designed for applications where media compatibility and low trapped volume are important. They are designed for applications like infusion pumps, wearables, drug delivery systems, and robotics, and are temperature compensated and calibrated internally.
It’s noteworthy that they do not do internal amplification of their signal, so the resolution is infinite. Instead, designers can leverage this unamplified signal to get the maximum resolution required per the application from their range of 100 kPa to 1 MPa.
Other design considerations
While pressure sensors are being designed to satisfy new engineering requirements of the IoT, they still need to address traditional issues like ruggedness and resistance to chemicals such as chlorine, bromine, and salt water. The moisture sensitivity level is another critical challenge that goes beyond the protection of the pressure sensor electronics. Additionally, pressure sensors should also be easy to install and maintenance free.
Figure 7: The surface mount NPA series from Amphenol provide flexible output options. (Image source: Amphenol)
The NPA series is available in gauge, absolute and differential pressure ranges, with either millivolt, amplified analog, or digital outputs. Their pressure measurement range is given as 10 inches of water (H20) (1 inch H20 = 249.0889 Pa) to 30 psi (1 psi = 6894.7529 Pa).
Pressure sensors are an important building block for many IoT applications. As the cost of IoT devices fall, form factors shrink, and time to market pressures increase, sensor manufacturers have responded with improved and more adaptable sensing and compensation capabilities, as well as simplified interfaces.